1979, the film Alien: “In space, no one can hear you scream.”
As you might expect, Y is generally limited to things a person can emit, usually with his voice. E.g., pray, yawn, groan, sing in the shower. (All of these turn up on a Google search for "in space no one can hear you".) Y does occasionally appear with a non-vocal meaning, but I believe this is a violation of snowclone-hood, because the further it gets from having something in common with scream, the less likely it is to evoke the movie quote. I think in order for a phrase to be a true snowclone, it will remind the hearer of its original source.
X also seems to be somewhat limited. If it is not some variation on space (Mars, cyberspace) then Y is much more likely to be scream, so as to keep the Alien reference unambiguous.
This is one of the first instances of a snowclone written about by Geoff Pullum, before had even come up with the word “snowclone.”
Hat tip to commentator Jeremiah for pointing out the other variable.